We concur with Dr. Sellier and colleagues that loss to follow-up and the unavailability of viral load data for all patients limited our study. Given the improvements in technology, viral load testing became routine among infected pregnant women only after July 2007. In addition, as in any prospective study, loss to follow-up is inevitable. We acknowledged these points in the limitation section, but, to our knowledge, our study nevertheless represents the largest examination to date of the effectiveness of hepatitis B virus immunoprophylaxis in which viral load data were available.

We also agree with Dr. Sellier and colleagues' recommendation for additional research. Such research, however, requires efforts to identify a target population that is likely to potentially benefit from interventions and, at the same time, excludes populations that are unlikely to benefit from participating in costly intervention studies. Thus, although risk for transmission and viral load probably have a linear relationship as Dr. Sellier and colleagues note, we believe that it is reasonable within the context of our study to identify a viral load below which no transmission occurred within our population. This finding allows future researchers to have potential anchor points for inclusion and exclusion criteria for studies that might evaluate whether such agents as nucleoside analogues benefit those who still have residual risk for vertical transmission of hepatitis B virus.